1. Field of the Invention
The present invention relates to an image processing apparatus, method, and program for extracting a shadow region in an image, and more particularly to an image processing apparatus, method, and program for extracting a shadow region from each of a plurality of contrast enhanced three-dimensional images obtained at different time points.
2. Description of the Related Art
In the medical field, it has been practiced to extract and display a shadow region representing a tumor region or the like in a contrast enhanced three-dimensional image in order to reduce the burden of radiological reading on doctors or the like.
For example, a photography method called dynamic CT that combines contrast agent angiography with X-ray CT is used, in which a plurality of contrast enhanced three-dimensional images is obtained by injecting a contrast agent into a subject and serially photographing the subject.
In the dynamic CT, the photographing is performed serially within a few minutes from the time when a contrast agent is injected into a subject to the time when the agent permeates over the entire liver after flowing through artery and portal vein. Here, it is necessary to perform comparative reading of a plurality of contrast enhanced three-dimensional images for temporal changes. Accordingly, the amount of reading required for one patient is increased to several times of that of ordinary photographing.
Further, the amount of data obtained at a time has increased dramatically with the advancement in accuracy of CT systems, so that the burden of radiological reading has become a serious problem. Under such circumstances, it is urgently required to improve the environment that allows a doctor or the like to perform an accurate diagnosis of a liver in a short time without overlooking a tumor region in the liver, and there is a growing demand for the development of computer aided diagnosis system.
A literature “Liver Cancer Detection based on a Temporal Density Feature from Abdominal Dynamic X-ray CT Images” by Y. Wakida et al., Journal of Computer Aided Diagnosis of Medical Images, Vol. 10, No. 1, pp. 1-10, 2007 proposes a method for detecting a tumor region of a liver with respect to each of contrast enhanced three-dimensional images obtained at different time points.
The contrast patterns of contrast enhanced three-dimensional images obtained at different time points, however, show different features at different phases, such as early phase, portal phase, late phase, equilibrium phase, and the like. Generally, it is known that a tumor region of a liver shows a characteristic contrast pattern that it is dyed densely (high CT value) in the early phase, comparably in portal phase, and thinly (low CT value) in the late phase.
Therefore, it is difficult for the method proposed in the literature described above to detect a tumor region of a liver from a contrast enhanced three-dimensional image taken at a predetermined time point and cannot satisfy the accuracy level demanded by a doctor. Further, it requires a huge amount of time for non-rigid registration.
In view of the circumstances described above, it is an object of the present invention to provide an image processing apparatus and method capable of extracting a shadow region from contrast enhanced two-dimensional images constituting contrast enhanced three-dimensional images obtained at different time points in an improved manner. It is a further object of the present invention to provide a computer readable recording medium on which is recorded a program for causing a computer to function as the image processing apparatus.